Which of the following best describes the normal boiling point of a compound?

• A. The temperature at which the vapor pressure of the liquid equals 0.5 atm
• B. The temperature at which the liquid freezes
• C. The temperature at which the liquid's density equals that of the vapor
• D. The temperature at which the vapor pressure of the liquid equals 1 atm

Answer: (D)

Verified step by step guidance

1. Understand that the normal boiling point of a compound is defined as the temperature at which its vapor pressure equals 1 atmosphere (atm).
2. Recall that vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature.
3. Recognize that boiling occurs when the vapor pressure of the liquid matches the external pressure, allowing bubbles of vapor to form within the liquid.
4. Note that the 'normal' boiling point specifically refers to the boiling point at 1 atm pressure, which is standard atmospheric pressure at sea level.
5. Therefore, the correct description of the normal boiling point is the temperature at which the vapor pressure of the liquid equals 1 atm.